An electrophotographic system is a method in which printed material is obtained by uniformly charging the surface of an image bearing member such as a photosensitive member (charging step), forming an electrostatic latent image on the surface of the image bearing member by photoexposure (photoexposure step), developing the formed electrostatic latent image with a developer formed of colored resin particles (development step), transferring the developer image to a recording medium such as paper or plastic film (transfer step), and fixing the transferred developer image to the recording medium (fixing step).
The developers here are broadly classified into dry developers and liquid developers: colored resin particles constituted of a material that contains a binder resin and a colorant such as a pigment are used in a dry state in the former, while the colored resin particles are dispersed in an electrically insulating liquid in the latter.
The need for color output and high-speed printing from image-forming apparatuses that use an electrophotographic system, e.g., copiers, facsimile machines, printers, and so forth, has been increasing in recent years. Within the realm of color printing, the demand for high-resolution, high-quality images has resulted in demand for developers that can accommodate high-speed printing while having the ability to form high-resolution, high-quality images.
Liquid developers are known to be developers that offer advantages with regard to color image reproducibility. With a liquid developer, the occurrence of aggregation by the colored resin particles in the liquid developer is suppressed during storage, and due to this a microfine toner particle can be used. As a consequence, excellent properties with regard to the reproducibility of fine line images and the reproducibility of gradations are readily obtained with a liquid developer. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that exploit these excellent features by utilizing electrophotographic technologies that use liquid developers. In view of these circumstances, there is demand for the development of liquid developers that have even better properties.
Investigations have been carried out into the addition of lecithin to liquid developers in order to bring about electrophoresis-mediated development by and transfer of the developer through the stable dispersion and charging of the toner particles in the carrier liquid.
For example, PTL 1 indicates that lecithin effectively acts as an amphoteric emulsifying agent. PTL 2, on the other hand, indicates that the generation of charge on the toner particle is promoted by the addition of lecithin as a charging regulator. In addition, PTL 3 indicates that lecithin is a charge control agent.